With a crumbly shortbread base and fudgy caramel filling, these chocolate-topped bars make a perfect afternoon treat.
Heat the oven to 200°C/180°C fan/400°F/Gas 6.
To make the shortbread base, combine the flour, sugar and cornflour in a large bowl. Add the softened butter and rub it in until the mixture comes together in large clumps and no floury bits remain. Form the dough into a ball, then press it into the base of the prepared tin to form an even layer.
Prick the shortbread dough all over with a fork, then bake it for 20 minutes, until firm and lightly golden. Set aside to cool in the tin.
Meanwhile, make the filling. Put the condensed milk, butter alternative and sugar in a large saucepan. Cook over low heat, stirring until the sugar dissolves, then bring the mixture to the boil. Reduce the heat to low, and simmer, stirring all the time, for about 15 minutes, until the caramel reaches hard ball stage (120°C on the sugar thermometer).
Quickly stir in the salt and immediately pour the caramel over the shortbread and level the top. Leave to cool for 20–30 minutes, then transfer to the fridge to chill for 1–2 hours, until set.
Once the caramel has set, make the chocolate topping. Melt the chocolate and oil in a glass bowl set over a pan of gently simmering water. Pour the chocolate over the set caramel and shake the tin gently to level the top. Return the tin to the fridge for 30 minutes, or until the chocolate is set.
Carefully remove the millionaire’s shortbread from the tin and, using a hot, sharp knife, cut it into 16 equal squares. Sprinkle each square with a little flaky salt or gold leaf if you like.